The present invention relates to a material testing system that applies force loads to a test specimen. More particularly, the present invention relates to a test specimen holder that holds the test specimen in the material testing system.
Test specimen holders or grips are well known in the material testing art and are used frequently to hold a test specimen in a material testing system. One well known grip includes opposed jaws that grip the test specimen therebetween and slide upon converging surfaces of a support frame. These jaws grip the test specimen without altering the vertical position of the jaws on the test specimen, thereby making it possible to preselect the exact point at which the specimen will be held in order not to impart tension loads during placement of the test specimen and the material testing system. However, in order to hold the test specimen in place, substantial compressive forces must be applied to the test specimen from the jaws. When the test specimen is made from materials such as metals, compressive forces do not damage the test specimen. However, other test specimens are made from materials such as glass or ceramics which cannot accept substantial compressive holding forces. Thus, test specimen holders that incorporate jaws as described above cannot be used.
Commonly, test specimens made from brittle material such as glass or ceramics are elongated with enlarged ends. In the art, these specimens are known as "button-head specimens". Annular surfaces formed on the "buttons" are used to hold the test specimen and are used to apply test forces. One known assembly for holding a button-head test specimen in a material testing system includes mounting an adapter to a MTS 646 Hydraulic Collet Grip sold by MTS Systems Corporation of Eden Prairie, Minn. The adapter includes separable members that when joined together form a flange for engaging the annular surface of the button-head and an aperture through which the elongated portion of the test specimen can extend. The separable members are secured to the grip and to each other. The grip and the adapter do allow preload forces to be applied to the test specimen. However, when the test specimen is located in a temperature chamber, the preloading device is also located in the temperature chamber.